Dome restraint assembly for rocket motors

ABSTRACT

An adjustable restraint assembly connectable between a dome portion of a solid propellant rocket motor casing and a rigid base or support, for use during the curing of a propellant in the casing. The assembly includes an upper attach bracket connectable to the dome portion, a lower attach bracket connectable to a rigid base, upper and lower flanges connected to the upper and lower brackets, respectively, and an adjustable threaded turnbuckle-like member connected between the flanges. The assembly is used to establish a maximum net downward deflection of the dome portion upon maximum pressurization of the uncured propellant in the motor casing and is thereafter adjusted at various times during the curing process to reduce and ultimately eliminate the deflection in increments to thereby improve the bond between the grains of the cured propellant and the casing in the region of the forward dome.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or forthe Government of the United States for all governmental purposeswithout the payment of any royalties.

BACKGROUND OF THE INVENTION

This invention relates generally to an assembly and method for producinga maximum net outward deflection of a forward or aft dome portion of arocket motor during a solid propellant curing operation and for reducingsuch deflection in stages at different times during the curing process.

It has long been known that when a propellant is cured under highpressure in a solid propellant rocket motor, the propellant containingmotor casing tends to expand in a non-uniform manner. Similarly, whenthe pressure is reduced in the casing during the curing process, thecasing tends to contract in a non-uniform manner. Moreover, as thepropellant cures, it tends to shrink inwardly, sometimes resulting inhigh localized stress fields in the propellant grain at the boundary ofthe grain/case bond. High localized stress fields and disruptions of thebond between the propellant grain and motor casing have beenparticularly pronounced in the region of the forward and aft domes. Theproblem has been recognized as significant in high pressure cured solidpropellant motors having both relatively rigid steel casings as well asthose having flexible, filament wound casings such as the Boeing SRM-1and SRM-2 large and small inertial upper stage motors used in theshuttle orbitor program.

By means of my invention, the deleterious effects of this problem aresubstantially minimized if not altogether eliminated.

SUMMARY OF THE INVENTION

It is an object of my invention to provide an adjustable restraintassembly connectable between a dome portion of a rocket motor case and arigid base or support to produce a net outward deflection in the domeduring the process of curing a solid propellant therein and for reducingthe net outward deflection in increments as the curing pressure isreduced, to thereby minimize localized grain stress fields in the regionof the bond between the cured propellant and the dome portion of thecase.

It is a further object of my invention to provide a method for improvingthe bond between a cured solid propellant and a dome portion of a rocketmotor casing containing such propellant.

Briefly, in accordance with my invention, there is provided a domerestraint assembly for use in a solid fuel rocket motor during apropellant curing operation. The assembly includes first attaching meansconnectable to a dome of a rocket motor and a second attaching meansconnectable to a rigid base or support. Lastly, a means is adjustablyconnected between the first and second attaching means for selectivelyadjusting the spacing between the first and second attaching means.

These and other objects, features and advantages of my invention willbecome apparent to those skilled in the art from the detaileddescription and attached drawings upon which, by way of example, only asingle preferred embodiment of the invention is disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a side elevation view of a conventional solid propellantrocket motor disposed in a support stand of the type used during apropellant loading and curing process, each having parts torn away toexpose a forward motor dome restraint assembly representing onepreferred embodiment of my invention.

FIG. 2 shows an enlarged side elevation view of the restraint assemblyof FIG. 1.

FIG. 3 shows an oblique projection of certain parts of the restraintassembly of FIGS. 1-2.

FIG. 4 shows a graphical step function of pressure applied to a curingpropellant within a chamber of the motor of FIGS. 1-2 versus curing timein days and hours, illustrative of one preferred propellant curingmethod of my invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing figures, particularly FIGS. 1-3, there isshown, in one preferred embodiment of my invention, a conventional solidpropellant rocket motor 10 having a flexible, resilient, filament woundcase 12 suspended in a cylindrically shaped support stand 14 of the typeused during propellant loading and curing operations. The motor 10 ofthe present example includes a propellant reservoir chamber 16 disposedwithin an aft dome 18 which communicates by means of a port 20 with apropellant main chamber 22. Uncured, liquidous propellant is introducedinto the reservoir 16 and thereafter, the main chamber 22 through a port23 in the aft dome 18. Also included is a forward dome polar boss 24which is imbedded in the case 12 which contains a threaded centralopening therethrough in which a similarly threaded circular adapterplate 26 is tightly secured. A cylindrically shaped mandrel 28, which isremovably disposed within the case 12 during propellant loading andcuring operations, is securely attached to the aft dome 18, and onlyloosely connected to the plate 26 by means of a nut and bolt combination29. Accordingly, during the propellant loading and curing operation, theforward boss 24 and plate 26 will be movable downwardly away from theforward (lower) end of the mandrel 28 by at least a minimum preselecteddistance as hereafter more fully explained.

The motor 10 is suspended in the stand 14 from excess strands 30 offilamentary material left over after the case 12 has been wound. Endportions of the strands 30 are secured between a circular ledge 32formed around an interior wall of the stand 14 and an overlyingannularly shaped angle bracket or aft skirt 34. The skirt 34 is fastenedto the ledge 32 by means of a series of bolts 36 circumferentiallyspaced therearound.

Now in accordance with my invention, there is shown a forward dome orboss restraint assembly 38 connected between the forward polar bossadapter plate 26 and a suitably rigid base which, in the presentexample, consists of a single steel I-beam 40. The beam 40 is connectedto opposing base portions of the stand 14 and extends across the hollowinterior thereof. The assembly 38 includes an upper attach bracket 42connected to the plate 26 by suitable fasteners 44, an upper flange 46connected to the bracket 42 by fasteners 48, a lower attach bracket 50connected to the beam 40 by fasteners 52, a lower flange 54 attached tothe bracket 50 by fasteners 56, and an adjustable screw coupling 58connected between the flanges 46 and 54. The coupling 58 of the presentexample is a unitary element having a central portion formed in theshape of a hex-nut 60 for rotational adjustment by means of a wrench, anupper portion forming a first threaded shaft 62 which is inserted into asimilarly threaded collar 64 of the flange 46, and a lower portionforming a second threaded shaft 66 inserted into a similarly threadedcollar 68 of the flange 54. The threads of the shafts 62 and 66 arewound oppositely from one another as shown most clearly in FIG. 2 sothat, as the nut portion 60 is adjusted in one direction of rotation,the flanges 46 and 54 are drawn toward one another, and as the nutportion 60 is adjusted in the other direction of rotation, the flanges46 and 54 are moved away from one another. Assuming the beam 40 isrigidly fixed in position, adjustment of the nut portion 60 in onedirection will pull the flange 46, bracket 42, plate 26 and boss 24downwardly, while adjustment of the nut portion 60 in the reversedirection will force the same components to move upwardly.

Operation of the boss restraint assembly 38 during the process ofloading and curing propellant in a standard SRM-1 rocket motor, asrepresented in the present example by the motor 10, will now beexplained. Initially, the motor 10, with chambers 16 and 22 empty, islowered into the support stand 14 over the assembly 38 and secured tothe skirt 34 as previously discussed. The resulting assembly as shown inFIG. 1 is placed in the heating chamber of a suitable oven, not shown.The nut at 29 is loosened to permit the plate 26 and lower end of themandrel 28 to move apart vertically by about 0.30 inch. The fasteners 48are removed so that the upper flange 46 is supported entirely by thethreaded portion 62 and the hex-nut 60 is adjusted so that there existsa small gap between the upper flange 46 and the underside of the bracket42 of from 0.010-0.020 inch. Thus, the boss 24 will not be restrainedfrom being pushed downwardly during the subsequent operation of fillingthe chamber 22 with propellant.

Next, uncured liquidous propellant 69 is introduced through the port 23into the reservoir chamber 16 and thence into the main chamber 22through the port 20 under a casting pressure of about 25 psig. After thechamber 22 has been completely filled as evidenced by a residual levelof propellant in the reservoir chamber 16, the fasteners 48 are insertedand tightened in the flange 46 and the hex-nut 60 is adjusted toeliminate the gap between the upper flange 46 and attach bracket 42.This adjustment should be just sufficient to maintain the upper bracket42 and flange 46 in contact without causing any upward vertical orhorizontal twisting motion of the attach bracket 42 at this time orduring the oven baking and curing operation to follow.

Next, the hex-nut 60 is adjusted to induce a downward movement of theupper flange 46 and attach bracket 42 to, in turn, downwardly deflectthe plate 26 and forward polar boss 24. Ideally, the adjustment shouldbe just sufficient to deflect the forward boss 24 downwardly by anamount equal to the anticipated shrinkage of the propellant upwardlyaway from the boss 24 during the curing process to follow, minus anysignificant upward deflection of the support beam 40 resulting from thisadjustment. The chambers 16 and 22 are now pressurized through the port23 with nitrogen gas at a pressure of 280 psig +10 psig to begin thecuring process. As this high pressure is applied, the flexible case 12tends to expand radially outwardly toward the interior wall of the stand14 such that the forward boss 24 tends to be pulled upwardly by anamount dependent upon the stiffness of the beam 40. The goal is toobtain a net downward displacement of the boss 24 under the restraint ofthe assembly 38 during the period of application of the highest curinglevel pressure beyond what the downward displacement level of the boss24 would have been if the restraint assembly 38 had not been in use. Inthe case of a standard SRM-1 motor, the net downwardly deflection shouldbe 0.2 inch after the internal pressure in the chamber 22 has reachedits maximum value at the commencement of the curing process.

Referring now also to FIG. 4 and having satisfied the aforementionedrequirements, the 280 psig propellant cure level pressure is maintainedin the chambers 16 and 22 for about 13 days while the motor 10 issubjected to a constant oven temperature of about 140° F. At the end ofthis time period, the oven temperature is reduced to a motor storagetemperature range of 50°-70° F. and the gas pressure in the reservoir 16is reduced until the pressure in the chamber 22 has decreased to about255 psig as at 70 in FIG. 4. This condition is maintained for about 8hours at the end of which time period the pressure in the chamber 22 isreduced by a second 25 psig increment as at 72 to about 230 psig and thehex-nut 60 is adjusted with a wrench in approximately quarter turnincrements until the forward boss 24 moves upwardly a distance of from0.06 to 0.07 inches. This is the first of three such adjustments to bemade in the deflection level of the boss 24 and occurs at the positionmarked 73 in FIG. 4.

A pressure of 230 psig is thereafter maintained in the case 22 for 8hours to the 16 hour point in FIG. 4, at which time gas is again bledfrom the case 12 until the pressure in the chamber 22 falls to 205 psigas at 74. Eight hours later, at the 24 hour level, the pressure in thechamber 22 is again reduced by 25 psig to the 180 psig level as at 76and maintained at this pressure for an additional 8 hours. At the 36hour point, the pressure in the chamber 22 is reduced by 60 psig to 120psig as at 78, and the hexnut 60 is adjusted a second time in quarterturn increments until the boss 24 moves upwardly an additional 0.06-0.07inches, as at 79. Pressure in the chamber 22 is thereafter maintained at120 psig for 24 hours to the 60 hour point at which time the chamberpressure is reduced by a 50 psig increment to 70 psig, as at 80, and afinal adjustment of the nut 60 is made to completely remove the downwardloading on the boss 24 by the assembly 38 as at 81. To complete thepresent example, the chamber 22 is maintained at an internal pressure of70 psig for an additional 60 hours to the 120 hour point in FIG. 4 atwhich time the internal pressure in the case 12 is bled to 0 psig afterwhich the usual removal sequence for the reservoir 16 and mandrel 28 isconducted.

While it has been found preferable to loosen the boss restraint assemblyof my invention in several steps, as for instance, in three separatesteps as indicated in the previous example, a greater or lesser numberof incremental adjustments may be employed with varying degrees ofsuccess. Under the most ideal circumstances, of course, it would bepreferable, although perhaps impractical and unnecessary, to continuallyloosen the restraint assembly as shrinkage in the propellant occurs inthe vicinity of the forward dome of the motor during the curing process.

The net downward deflection of the forward dome polar boss to beestablished initially when using my restraint assembly on any givensolid fuel rocket motor during the propellant loading and curing processwill depend upon a number of factors including the size and shape of theparticular motor, the shrinkage characteristics of the particularpropellant used, and the stiffness of the base support means or beam towhich the restraint assembly is connected. Moreover, the rate at which agiven propellant is cured, its curing temperature and pressure levels,and deflection of the forward boss when unrestrained during the fuelloading process are additional factors which may or will have a bearingon the selection of initial downward deflection adjustments of theforward boss. Accordingly, in the case of each different motor,propellant and curing process employed, some experimentation will benecessary in order to determine the most desirable initial net downwarddeflection of the forward dome of the motor casing to be established forbest results. It will also be appreciated that my restraint assembly andmethod for its use may also be employed with beneficial results onrocket motors having relatively inflexible cases such as those made ofsteel.

Although the present invention has been explained with respect tospecific details of one preferred embodiment thereof, it is not intendedthat such details limit the scope of the foregoing claims otherwise thanas specifically set forth therein.

I claim:
 1. A support assembly for adjustably restraining the domeportion of a solid propellant rocket motor case during propellantloading and curing, comprising:first attaching means for connection tosaid dome portion; a substantially rigid base; second attaching meansconnected to said rigid base and a screw coupling connected between saidfirst and second attaching means for selectively adjusting the spacingbetween said dome portion and base.
 2. The assembly of claim 1, whereinsaid first attaching means comprises a U-shaped bracket, and furthercomprising means for attaching said bracket to said dome portion.
 3. Theassembly of claim 1 further comprising a first flange member connectedto said first attaching means, and a second flange member connected tosaid second attaching means, said screw coupling being connected betweensaid first and second flange members.
 4. A support assemby foradjustably restraining the dome portion of a solid propellant rocketmotor case during propellant loading and curing, comprising:firstattaching means for connection to said dome portion, and a first flangemember connected to said first attaching means; a substantially rigidbase; second attaching means connected to said rigid base, and a secondflange member connected to said second attaching means; adjusting meansconnected between the first and second flange members for selectivelyadjusting the spacing between said first and second attaching means,said adjusting means comprising a unitary element containing a centralportion forming an adjustable nut, an upper threaded portion extendingfrom one broad surface of said nut portion, and a lower threaded portionextending from the other broad surface of said nut portion, said upperand lower threaded portions containing screw threads wound oppositelyfrom one another, said first and second flange members containingcollars defining hollow shafts therein threaded in conformity with thethreads on said first and second threaded portions, respectively.
 5. Theassembly of claim 4 wherein said first attaching means comprises aU-shaped bracket, and further comprising means for attaching saidbracket to said dome portion.